Electric arc furnaces are widely used for the production of hot metal from iron and steel scrap and the overall objective in arc furnace melting is to produce hot metal of the desired quality at the lowest cost. By conducting meltdown, i.e. the melting of scrap, at optimum power levels and minimizing the time duration of interruptions of optimum power level operation during meltdown a maximum cost saving can be achieved since most of the energy required in producing hot metal product is used during meltdown. In a three phase electric arc furnace, the secondary circuit of a three phase transformer is connected to the three electrodes of the furnace so that each electrode is powered by a different phase of the secondary circuit, the powering phase voltage being the voltage from phase to neutral, i.e. ground, to which the furnace shell is electrically connected. At the beginning of the melting operation, i.e. melt down, metal scrap is charged to the furnace and the three electrodes descend toward the scrap charge. One of the electrodes first approaches the charge and stops and a second electrode approaches the charge and an arc is created at both electrodes. In this circumstance, the furnace is operating single phase, and, inefficiently, until the third electrode descends to create its arc and three phase furnace operation commences and scrap meltdown begins. In a typical commercial operation, an automatic regulator device is provided for each electrode and a set point voltage proportional to the phase voltage is selected for each regulator device to maintain a desired predetermined distance between each electrode tip and the scrap below. This distance (and the set point) is selected by the furnace operator based upon the type of charge material, available power input and other furnace parameters and is commonly about 100 to 300 mm. This selected pre-determined distance is maintained by the respective regulator devices during meltdown of the electrodes through the scrap by comparing the selected set point voltage with measured voltage and/or current information for each of the electrodes. The regulator operation proceeds routinely during melt-down to continuously maintain the predetermined distance between the electrode tips and the scrap below unless there is a disruptive occurrence, e.g. a "scrap fall" whereby scrap collapses inwardly toward and against the side of an electrode to thereby short circuit the electrode. Under such circumstances, current will increase substantially in the short-circuited electrode and also in an adjacent electrode. With conventional regulating systems the regulators of the thus affected electrodes will operate to rapidly raise these electrodes to quickly reduce current in the electrodes and then re-establish the pre-selected distance between the electrode tip and the scrap below, based on the initially selected set point voltage. While the short circuit due to scrap-fall persists the real power, MW, applied to the scrap charge is substantially lessened and the result is that the average real power applied to the furnace charge during meltdown is reduced and the cost of furnace operation correspondingly increased.
In the situation where a scrap fall below the electrodes increases the distance from an electrode tip to the scrap so that the arc is extinguished, the particular electrode is open circuit and three phase operation is interrupted and the furnace operation is single phase, using only two arcs and there is a substantial drop in power delivered to the change and furnace operation is thus highly inefficient. With conventional regulating systems, the regulator of the extinguished electrode operates to quickly lower this electrode to re-establish the pre-selected distance between the electrode tip and the scrap below based on the initially selected set point voltage. While the extinguished arc condition due to scrap fall persists the real power, MW, applied to the scrap charge is substantially lessened and the result is that the average real power applied to the furnace charge during meltdown is reduced and the cost of furnace operation correspondingly increased. It is accordingly an object of this invention to provide a regulation system for minimizing the reduction which occurs due to "scrap fall" at an electrode during meltdown.